Tonle Sap (pronounced tone-lay sop) is the largest lake in Southeast Asia. The river that connects the lake to the Mekong River, the Tonle Sap River, has the distinction of being the world’s only river that reverses course twice each year flowing in opposite directions.

In 2015, I spent a few days in Siem Reap, Cambodia exploring the city and visiting the temple complexes of Angor Wat and Angor Thom. Siem Reap is situated only a few miles northeast of Tonle Sap. When I learned of the presence of this remarkable body of water, I quickly decided to book a tour.

I came across the photos recently and thought I’d share them. I don’t have notes and memory has faded to an extent. The commentary in italics is quoted from an online article “Tonle Sap – The Beating Heart of Cambodia”.


As Southeast Asia’s largest fresh body of water, Tonle Sap Lake acts as the central lifeline for over 1.2 million Cambodians living along its banks. In addition to hosting the most productive inland fishing on the planet, the similarly named Tonle Sap River has the distinction of being the world’s sole waterway which runs in both directions.




That is to say, its flow of water reverses every six months out of the year, causing the lake to swell to an astonishing five times its size between the wet and dry seasons. And like a living, breathing organism, its residents have skillfully adapted to the Tonle Sap’s rhythmic, annual inhalation of water, flora and fauna.

The tour visited a floating school with classes in session. Children were glad to have visitors and seemed accustomed to the intrusion.



Visitors can purchase large sacks of rice. The rice which is bought is cooked for the children at the school.



During the dry season, which typically lasts from November to May, the Tonle Sap flows towards the sea, much like any other stream, in a south-easterly direction. Roughly 100 km (62 mi) downstream, the Tonle merges with the mighty Mekong River, which then branches off into several waterways throughout Vietnam’s thriving Mekong Delta region and into the South China Sea.


Unlike the floating markets in Thailand, the whole community floats at Tonle Sap. Transportation for the locals is by tiny wooden canoes.


They share the waterways with tour boats and commercial traffic.


The little canoes somehow manage to avoid being swamped by the wakes of larger, faster craft.


Over the wet season, however, the Mekong River becomes so inundated with torrential rains throughout Southeast Asia in addition to the melting snows from Tibet (4,500 km/2,800 mi upstream), that its excess waters take an alternate, north-western route up the Tonle Sap River.


In turn, Tonle Sap Lake expands from an area of 2,700 to 16,000 sqkm (1,050 to 6,200 sqmi, see map above-left) at the peak of the wet season, while its widest point grows from 35 to 105 km (22-65 mi). And because the depth of the Tonle Sap rises so drastically, from roughly 1-metre to 9-metres, lakeside structures are built to adapt in one of two ways: houses that either float or stand on stilts up to 10-metres (33 ft) high.




Though dry season means the shoreline recedes from underneath the floorboards to tens of kilometres away, the muddy Mekong waters leave behind rich soil deposits ideal for rice cultivation. Thus, fisherman all across the wetlands hang up their nets in lieu of farming equipment for half the year.


Not only does the Tonle Sap provide for those in its immediate vicinity, but it also acts as both a safety valve and reservoir for those living further downstream in Vietnam’s Mekong Delta. Unlike the Red River Delta in northern Vietnam, which is quite prone to natural disaster, the Tonle Sap’s ability to absorb excess water prevents flooding in the southern Mekong Delta during the heavy rains. Conversely, drainage from the Tonle Sap contributes half of the Mekong’s water flow during dry months.


I thought the tour was very interesting. Others not so much.


It is easy to join a Tonle Sap tour in Phnom Penh or Siem Reap. They can be booked online or at any of the ubiquitous street side kiosks that sell tickets for tours and attractions. Half-day tours from Siem Reap go for as little as $20 per person.

Overall Impression

I very much enjoyed the tour of Tonle Sap and getting a glimpse of the lifestyle of the people who live on the lake and its waterways. Those people serve as an example of how life finds a way to adapt to unusual circumstances. I imagine that Tonle Sap’s bounty was likely a factor in the development of the civilization which constructed huge, elaborate temple complexes at Siem Reap, a few miles from its northeastern shore.

I recommend visiting Tonle Sap on your next trip to Southeast Asia. The ideal time would be in May or November when the river changes directions. The precise moment that happens is probably difficult to predict.